This invention relates to a method of manufacturing high tensile strength steel plates, in particular to a method of manufacturing steel plates with thickness over 25 mm and tensile strength over 80 kg/mm.sup.2.
There is a strong need for steel plates with tensile strength over 80 kg/mm.sup.2, with high notch toughness and good weldability. The prior art steel of these strength levels were manufactured by reheat-quenching and tempering process. However, so-called direct quenching process, wherein a steel plate is quenched immediately after hot rolling, has been recently introduced to plate production.
The direct-quenched steel exhibit higher hardenability compared with that of conventionally reheat-quenched steel with the same chemical composition. By utilizing the beneficial effect of direct-quenching process, the amount of the alloying elements can be decreased which leads to the improvement in the weldability.
However, the prior art direct-quenching process has a disadvantage in obtaining uniform mechanical properties along the longitudinal and thickness directions. Thus, it is still difficult to manufacture a plate by the direct-quenching process which satisfies the recent increasing demand for the high toughness at any portion within the plate.
The nonuniformity along the longitudinal direction is caused by the conventional cooling method in which quenching is carried out continuously. According to the continuous quenching method, the plate is quenched continuously from its head to tail end by passing the plate through a relatively short cooling zone with high cooling water density. According to this method, it takes more than few minutes to quench the entire length of the plate, thus cause variation in the time to start quenching after rolling along the longitudinal direction. During the duration time, recovery and recrystalization of austenite would occur together with the temperature drop of the steel plate. Such change in the austenite condition and temperature along the plate will result in the nonuniformity of the mechanical properties along the longitudinal direction.
Such problem can be avoided by using static cooling method, wherein the plate is placed in the cooling zone longer than the plate and perform quenching of the entire length of the plate simultaneously. The reason why the continuous quenching method has been nevertheless adopted in most mill is that it was believed that the high quenching rate is necessary for the improvement in mechanical properties and weldability.
The quenching rate increases as the cooling water density (amount of water flow per unit time and unit area) increases. On the other hand, the total amount of water available for in-line quenching is limited. Consequently, the length of the quenching zone has to be limited in order to obtain high water density and thus high quenching rate. When the length of the quenching zone become shorter than the length of the plate to be processed, then the continuous method has to be adopted.
The nonuniformity in the thickness direction is caused by the difference in the cooling rate between the surface and the core portion of the plate. The difference is enhanced when the thickness of the plate and/or the water density increases. The difference in cooling velocity results in the variation in the resultant micro-structure of the steel and thus the inhomogeneity in mechanical properties.
Such problem had been recognized already and some ideas to solve the problem had been proposed.
For example, Japanese patent laid-open publication No. 101613/1977 discloses a method for decreasing the difference in cooling velocity between the vicinity of surface and the core portion. According to this method, the steel plate is passed through strong cooling zone and soft cooling zone provided alternatively. However, this method can be applied only for the continuous quenching, thus the inhomogeneity in the longitudinal direction can not be avoided.
The problem becomes significant when the thickness exceeds 25 mm and the tensile strength exceeds 80 kg/mm.sup.2.
Such high strength steel exhibits the optimum strength and toughness when it has a mixed structure of martensite and lower bainite. When either the alloy content or cooling rate is too high, then the micro-structure after quenching becomes single martensite phase, and the toughness degrades. When either two is too low, then upper bainite will be included, and both toughness and strength degrades.
In other words, there is an optimum quenching rate for given chemical composition of a steel. Therefore, where there is a large quenching rate distribution in the thickness direction of the plate, it becomes impossible to obtain an optimum micro-structure and thus the best mechanical properties throughout the entire thickness.